1. Field of the Invention
The present invention relates to rotary head type magnetic recording/reproduction apparatuses such as helical scanning system video tape recorders (VTR), and more particularly, to a rotary head type magnetic recording/reproduction apparatus employing a dynamic tracking system in which a magnetic head is displayed in a track width direction by an actuator.
2. Description of the Background Art
In recent years, there have been efforts to significantly increase the density of recorded information in a magnetic recording/reproduction apparatus, resulting in reduction of the track width. Such reduction of the track width causes strict conditions on a magnetic head to carry out a satisfactory tracking operation. More specifically, when a signal is to be reproduced from a track having the width reduced in a conventional type of magnetic recording/reproduction apparatus having the magnetic head fixed to the rotary drum so that the magnetic head cannot move in the direction of the track width, deviation occurs in the tracking of the magnetic head, leading to a possibility that a sufficient reproduced output cannot be obtained from the magnetic head.
A magnetic recording/reproduction apparatus including a dynamic tracking system is proposed, such as in U.S. Pat. No. 4,237,399. Such a magnetic recording/reproduction apparatus uses a magnetic head that is displaceable in the direction of the track width by an actuator, in which the position of the magnetic head in the track width direction is controlled by a closed loop according to a signal reproduced from the magnetic head.
FIG. 1 is a block diagram schematically showing the main components of a dynamic tracking system of such a conventional magnetic recording/reproduction apparatus. The conventional dynamic tracking system of FIG. 1 includes a magnetic head 4, a reproducing circuit 5, a position error detection circuit 6, an adder 1, an actuator driving circuit 2, and an actuator 3.
In a reproduction operation, magnetic head 4 picks up a signal recorded on a magnetic tape (not shown), which is output as a signal x to the outside as well as to reproducing circuit 5. At the time of reproduction, reproducing circuit 5 generates a reproduced signal v according to output signal x of magnetic head 4. Position error detection circuit 6 detects an error signal e indicating the relative position error amount of magnetic head 4 with respect to a track on the magnetic tape according to output v of reproducing circuit 5. Error signal e is added with a reference driving signal a which is supplied from a controller (not shown) of the magnetic recording/reproduction apparatus and normally takes a value of zero in adder 1. The result of such addition is provided to actuator driving circuit 2 as a control signal b. Actuator driving circuit 2 is formed mainly of amplifiers (not shown) provided for loop gain adjustment or the like, and various filters provided for servo characteristic compensation or the like. According to control signal b, actuator driving circuit 2 generates a driving signal c which is provided to actuator 3. Actuator 3 responds to driving signal c to displace magnetic head 4 in the direction of the track width. A bimorph type piezo actuator using a piezoelectric element, or an electro magnetic type voice coil motor type actuator are generally used as actuator 3. The circuit configuration from actuator driving circuit 2 to adder 1 via actuator 3, magnetic head 4, reproducing circuit 5 and position error detection circuit 6 forms a closed loop control system.
Detection of error signal e by position error detection circuit 6 can be carried out in many ways. A typical one is a pilot signal method. According to this pilot signal method, the position error amount of a magnetic head and the polarity indicating the direction of deviation of the magnetic head from the track can be detected simultaneously by recording a pilot signal having a different frequency to be superimposed on a main signal for each track. This pilot signal method is also used in an automatic track finding (ATF) system employed in a 8 mm VTR. In the case of a 8 mm VTR, four types of pilot signals each having a frequency differing from each other are used.
FIG. 2 is a schematic diagram for describing the detection principle of a head position error amount at the time of reproduction by a position error detection circuit 6 when such 4 types of pilot signals are used (a 4-frequency pilot signal method). Referring to FIG. 2, a plurality of tracks 11 are formed on a magnetic tape 10. A magnetic head A traces these tracks. The width of magnetic head A is greater than 1 track pitch so that signals can also be reproduced partially from tracks adjacent to the currently traced track.
It is assumed that four types of pilot signals having different frequencies of F.sub.1, F.sub.2, F.sub.3, and F.sub.4 are recorded cyclically to be superimposed on a main signal in the plurality of tracks 11 respectively, as shown in FIG. 2. The pilot signal frequencies of F.sub.1, F.sub.2, F.sub.3, and F.sub.4 are defined as set forth in the following with the horizontal synchronizing frequency of a video signal being represented as f.sub.H.
f.sub.1 =6.5 f.sub.H PA1 f.sub.2 =7.5 f.sub.H PA1 f.sub.3 =10.5 f.sub.H PA1 f.sub.4 =9.5 f.sub.H
It is appreciated from the above definition of the pilot signal frequency and the arrangement shown in FIG. 2 that the difference in the pilot signal frequency between adjacent tracks is set so that 3f.sub.H and f.sub.H are arranged in an alternate manner such as 3f.sub.H, f.sub.H, 3f.sub.H, f.sub.H, 3f.sub.H, . . . .
In an azimuth type magnetic recording/reproduction apparatus, adjacent tracks are formed by heads having azimuths differing from each other at the time of recording. Therefore, in a reproduction operation using a magnetic head A having a width greater than 1 track pitch as shown in FIG. 2, the partially reproduced output from a track adjacent to the track currently traced by magnetic head A is greatly reduced in level with respect to the main signal component in the high frequency band due to azimuth loss. However, since the azimuth loss is small with respect to pilot signal components having the frequency band set to a low range, a large crosstalk signal is obtained as the partially reproduced output from an adjacent track.
According to the state shown in FIG. 2, magnetic head A tracing a track on which a pilot signal of frequency f.sub.2 is recorded reproduces the pilot signal components of frequencies f.sub.1 and f.sub.3 from the two adjacent tracks as crosstalk signals. By multiplying the crosstalk signals of frequencies f.sub.1 and f.sub.3 reproduced from the left and right adjacent tracks respectively by the pilot signal of frequency f.sub.2 reproduced from the reference track, two pilot beat signals can be obtained as the crosstalk components from the left and right adjacent tracks. By taking the difference between these two crosstalk components, the amount of deviation from the track being traced by magnetic head A can be determined by the level of that difference, and the direction of deviation can be detected according to the polarity thereof. Thus, the difference between two crosstalk components (pilot beat signals) is calculated by position error detection circuit 6 to be output as an error signal e.
However, the 4-frequency pilot signal method shown in FIG. 2 requires four types of signal generation circuits in order to generate pilot signals having four different frequencies of f.sub.1, f.sub.2, f.sub.3, and f.sub.4. There was a problem that the circuit complexity is increased.
To solve this problem of the 4-frequency pilot signal system, an intermittent track recording method using two types of pilot signals having different frequencies is proposed. Such a method is disclosed in, for example, Japanese Patent Laying-Open No. 60-25046.
FIG. 3 shows a format of such an intermittent track recording method using two types of pilot signals. Referring to FIG. 3, two types of pilot signals having different frequencies of f.sub.1 and f.sub.2 are alternately recorded for every other track. By tracing each of the tracks in FIG. 3 using a magnetic head A having a width greater than 1 track pitch as described before, crosstalk signals of the pilot signal components of frequencies f.sub.1 and f.sub.2 will be obtained from both the adjacent left and right tracks only when a track having no pilot signal recorded is traced as shown in FIG. 3. In other words, position error information of a magnetic head can be obtained for only every other track by the reproducing system of a single head shown in FIG. 3.
A conventional magnetic recording/reproduction apparatus employing such an intermittent track recording method uses a pair of magnetic heads as shown in FIG. 4 to overcome this disadvantage. More specifically, a pair of actuators 21 and 22 are provided facing each other by 180.degree. in the circumferential direction of a drum 20. A pair of magnetic heads A1 and A2 are disposed in close proximity to each other on one actuator 21, and a pair of magnetic heads B1 and B2 are disposed in close proximity to each other on the other actuator 22. The above-described problem is to be solved by tracking two adjacent tracks simultaneously by such pair of heads.
According to an azimuth recording system format, heads A1 and A2 of one pair are in opposite azimuth, and also heads B1 and B2 of the other pair are also in opposite azimuth. Therefore, these pairs of heads are called double azimuth heads.
FIG. 5 schematically shows the recording/reproduction principle using the double azimuth head of FIG. 4. In a recording operation, one pair of magnetic heads A1 and A2 forming a double azimuth head records main signals (each including a luminance signal and a chrominance signal) on one pair of adjacent tracks 11a and 11b. Simultaneously, a pilot signal of frequency f.sub.1 is recorded to be superimposed on the main signal on one track 11a by one magnetic head A1. When tracing of the track by heads A1 and A2 ends, the other pair of magnetic heads B1 and B2 (not shown in FIG. 5) forming the other double azimuth head records main signals on the other pair of adjacent tracks 11c and 11d. Simultaneously, a pilot signal of frequency f.sub.2 differing from frequency f.sub.1 is recorded to be superimposed on the main signal on one track 11c by one magnetic head B1. By repeating such recording operations cyclically, two types of pilot signals having different frequencies of f.sub.1 and f.sub.2 are recorded alternately for every other track.
In a reproducing operation, always magnetic head A2 (or B2) of one pair of magnetic heads A1 and A2 (or B1 and B2) forming the double azimuth head reproduces crosstalk signals of the pilot signal components of frequencies f.sub.1 and f.sub.2 from the left and right tracks, as shown in FIG. 5. Here, because one magnetic head A2 (or B2) that reproduces a crosstalk signal is fixed in close proximity on the same actuator 21 (or 22) with the other magnetic head A1 (or B1) that does not reproduce a crosstalk signal as the double azimuth head, similar tracking can be carried out integrally for the other magnetic head A1 (or B1) by carrying out closed loop control based tracking of one magnetic head A2 (or B2) that reproduces a crosstalk signal. Therefore, the heads can be positioned at high accuracy as a whole.
FIG. 6 schematically shows the installed manner of a conventional double azimuth head with respect to the actuator. On one actuator 21, a pair of magnetic heads A1 and A2 are installed with a difference in the vertical direction (in the height direction) corresponding to 1 track pitch Tp between magnetic heads A1 and A2. Similarly, on the other actuator 22, the other pair of magnetic heads B1 and B2 are installed with a difference in the vertical direction therebetween corresponding to 1 track pitch Tp.
However, such a provision of a pair of magnetic heads on the same actuator with a difference of Tp therebetween as shown in FIG. 6 requires significantly high accuracy due to reduction of a track width, i.e., reduction of one track pitch Tp in accordance with increase of density of recorded information as described before. It is extremely difficult to satisfy such a requirement of high accuracy, resulting in reduction of the manufacturing yield of a head drum. This causes increase in the manufacturing cost of a magnetic recording/reproduction apparatus.